1. Field of the Invention
The present invention relates to a toner image height measurement apparatus for measuring the height of a toner image carried on an image carrier, and relates to an image forming apparatus, such as a copying machine, a laser printer, or a facsimile machine, mounted with the toner image height measurement apparatus.
2. Description of the Related Art
Conventionally, there are mainly the following two types of electrophotographic full-color image forming apparatuses. The first one is called a four-cycle type in which an imaging apparatus having one photosensitive member is provided and toner images of plural colors are sequentially formed on the photosensitive member by an electrophotographic process according to image information and then transferred onto a transfer member so as to be superimposed one upon another, while rotating the transfer member which is in contact with the photosensitive member. Another one is called a tandem type in which imaging apparatuses each having one photosensitive member are provided and toner images of plural colors are formed on respective ones of the photosensitive members of the imaging apparatuses by an electrophotographic process according to image information and then sequentially transferred onto a transfer member so as to be superimposed one upon another, while rotating the transfer member which is in sequential contact with the photosensitive members. In most of these types of image forming apparatuses, an intermediate transfer member has recently been used in the transfer of toner images onto the transfer member.
With the above image forming apparatus, even if an image is formed under the same setting condition, the density of the formed image varies due to variations in image forming parameters such as toner charge amount, sensitivity of photosensitive member, and transfer efficiency and due to variations in environmental conditions such as temperature and humidity.
Conventionally, therefore, the density or the height of a toner image developed on the photosensitive member or on the intermediate transfer member is detected, and based on a detection result, toner replenishment and/or image forming parameters such as charge potential, image exposure light amount, and developing bias are controlled.
A technique for detecting the toner image density or the toner image height is disclosed in, e.g., Japanese Laid-open Patent Publication Nos. 4-156479, 8-327331, and 2007-199591. In the following, this technique will be described in brief with reference to FIGS. 14 and 15.
FIG. 14 schematically illustrates the toner thickness detection method.
First, a toner patch (toner image) 105 is formed on an image carrier 106 such as a photosensitive member or an intermediate transfer member. Then, light from a light source 101 such as a laser diode is collected on a surface of the toner patch 105 by alight collecting lens 102. Light reflected from the toner patch 105 is received by a light receiving lens 103 and formed into an image on a line sensor 104 that picks up a spatial intensity distribution (light intensity distribution) of the formed image.
Although not illustrated in FIG. 14, the image picked up by the line sensor 104 (i.e., a reflection waveform obtained from the toner patch 105) is converted into a digital signal, to thereby obtain reflection waveform data which is then stored in a memory and from which an amount of toner adhesion is determined by a signal processing unit. In the following, a method for determining the amount of toner adhesion is described.
FIG. 15 illustrates reflection waveforms obtained by the line sensor 104.
As shown in FIG. 15, reflection waveforms each having a peak near the center position on the line sensor 104 can be obtained by the line sensor 104. Since a light path length from the light source 101 to an upper surface of the toner patch 105 differs from a light path length from the light source 101 to an upper surface of the image carrier 106, a position on the line sensor 104 where an image of reflection light from the surface of the toner patch 105 is formed differs from a position where an image of reflection light from the surface of the image carrier 106 is formed. In other words, a difference between the positions of the peaks of the reflection waveforms (toner reflection waveform 201 and image carrier reflection waveform 202 in FIG. 15) varies according to the thickness of the toner patch 105, and therefore, the thickness of the toner (i.e., the amount of toner adhesion) can be measured from the difference (shown at Pd in FIG. 15) between the peak positions of the reflection waveforms.
The above method can be used effectively for color toner images (yellow, magenta, cyan, etc.) from which reflection light having a sufficient intensity can be obtained. On the other hand, for a black toner image, especially, for a high-density black toner image, a problem is posed that the toner image height cannot be measured with accuracy for the reason that a sufficient amount of reflection light cannot be obtained, as will be described below with reference to FIG. 16.
FIG. 16 is a graph showing a distribution of light amount (reflection waveform 201) obtained by the line sensor 104 in a case where the height of a black toner image is detected by the above described method.
Since most of light irradiated from the light source 101 is absorbed by the black toner, the intensity of a signal obtained by the line sensor 104 becomes extremely small as shown by the reflection waveform 201 in FIG. 16. It is therefore difficult to accurately identify the position of the peak of the reflection waveform 201, and the toner image height cannot be detected with high accuracy.